llvm dev - Nov 2011 - [LLVMdev] loop transformation: uniquely identifying a loop within a program

hi all,

We are writing a loop transformation using the llvm loop pass class, but we
do not want to run the pass on all loops within a program. We are looking
for suggestions on how we can uniquely identify certain loops that we want
to transform. So far, we are working on a suggestion to transform the loops
depending on which function they're run from in the program. Which will be
something like this:

   ...
   Function *F = L -> getParent() -> getParent();
...
   if(F->getName == "my_function") {
   //transform loops in my_function
   }

The function name can easily be passed in from the command line and this
would work fine for loops within that function only. But how can we extend
this for nested loops for example?

Thank you,

tatenda
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On Nov 10, 2011, at 2:39 AM, Tatenda M. Chipeperekwa wrote:
> hi all,
> 
> We are writing a loop transformation using the llvm loop pass class, but we
do not want to run the pass on all loops within a program. We are looking for
suggestions on how we can uniquely identify certain loops that we want to
transform. So far, we are working on a suggestion to transform the loops
depending on which function they're run from in the program. Which will be
something like this:
> 
>    ... 
>    Function *F = L -> getParent() -> getParent();
>    ...	
>    if(F->getName == "my_function") {
>    //transform loops in my_function
>    }
> 
> The function name can easily be passed in from the command line and this
would work fine for loops within that function only. But how can we extend this
for nested loops for example?
you can use if (L->getHeader()->getParent()->getNames() ==
"special_function") check, which will work for all loops.

-
Devang

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hi,

thank you for the response, we managed to get all the loops in a given
function using this:

> you can use if (L->getHeader()->getParent()->getNames() =>
"special_function") check, which will work for all loops.
>
>
As part of the transformation we need to get information about the loops,
and we are using the LoopInfo pass:

LI = &getAnalysis<LoopInfo>();

to get information such as the depth of the loop. We are also manipulating
the IV in some instances e.g. changing the step from say 1 to 2, and we
have implemented this by iterating through all the instructions in a BB. It
seems that the best approach to do this would be to use the
ScalarEvolution, which we have added to our pass:

SE = getAnalysisIfAvailable<ScalarEvolution>();

We been going through the LoopStrengthReduce pass but are unable to
implement this correctly, and to re-write the IR for these cases. Any
pointers on how to do this are appreciated.

Regards,

tatenda
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